The relationship between magnetic susceptibility and ferromagnetic minerals in rock is well established. Although this relationship varies with mineral assemblage, rock type, and with grain size, shape and orientation, there is in almost all cases, a strong sometimes nearly linear correlation between the magnetic susceptibility and ferromagnetic mineral content. Well-logging measurements of magnetic rocks that contain ore grade concentrations of iron minerals are possible with relatively simple, low-sensitivity logging probes. However, measurements become increasingly difficult as the ferromagnetic mineral content decreases to the low levels generally associated with sedimentary rocks (less than 0.1% magnetite). In order to measure the magnetic susceptibility of these rocks, the sensitivity of the well-logging system must be increased by several orders of magnitude. Consequently, noise and drift due to temperature variation and the mechanical stress on the components in the measurement system become significant. Nevertheless, borehole measurements of magnetic susceptibility of rocks that have low concentrations of ferromagnetic minerals is desirable because these measurements may reveal alteration zones associated with the emplacement of valuable non-ferrous minerals, particularly uranium in roll-type deposits.
In general, magnetic susceptibility in a borehole is measured by detecting changes in the effective inductance of a solenoid. This inductance is a function of the number of turns and the diameter of the wire wound in a helix about a core, and the properties of the core and all of the material surrounding the entire solenoid. All of these factors are temperature dependent.
Prior U.S. Pats. which describe the measurement techniques for magnetic susceptibility logging systems include U.S. Pat. Nos. 2,623,923 (Zimmerman), 2,625,583 (Broding) and 3,555,409 (Atwood et al). The measurement techniques disclosed in these patents are made without reference to temperature stability or the reduction of measurement drift. U.S. Pat. No. 2,640,869 (Zimmerman) discloses a temperature compensated susceptibility logging system. This system uses the temperature change in the resistance of one arm of a Maxwell bridge to compensate for variation caused by temperature changes in the measurement solenoid of the system in another arm of the Maxwell bridge. Maintaining the balance in this system is very difficult and thermal drift has still been unacceptable. A probe having a resistance heater surrounding the sensing coil was manufactured and sold by Simplec Manufacturing Company of Dallas, Tex., in 1971. The heater of the probe was thermoregulated through use of a transistorized power control circuitry using a thermistor to sense solenoid temperature and to feed back a bias voltage to the thermoregulator. A disadvantage is that the heater in the device interferes with the electromagnetic properties of the sensing solenoid.
In the early development and testing of prototype magnetic susceptibility well-logging probes by Broding and others, it was shown that changes in the magnetic susceptibility of rock were accompanied by nearly proportional changes in the amplitude of the quadrature phase of a bridge output signal of the probe. Thus, it was not difficult to design a measurement system with sufficient sensitivity to detect changes as small as one micro cgs unit, representing approximately 4 ppm. magnetite. However, it has been extremely difficult to stabilize the drift and reduce the noise so that borehole-logging measurements could be made at high sensitivities. Tests have shown that without any form of temperature stabilization, the drift has commonly exceeded 1,000 micro cgs units per hour. In addition, noise levels have been detected which have been as large or larger than the anomalies of interest (20 micro cgs units). While the Simplec design represented a significant improvement over previous systems, drift and noise are still too high to make accurate and reliable measurements of magnetic materials.
In U.S. Pat. No. 2,615,956 (Broding), a well-logging system is disclosed in which variations in the impedance of the cable, resulting from temperature variations to which the cable is subjected as it is lowered into the bore hole, are eliminated in the bridge measuring network, this being accomplished using frequency modulated signals. It has also been disclosed in U.S. Pat. No. 3,831,082 (Mazzagatti) that a monitoring system for the mud used in well drilling can be employed to provide a log of the magnetic susceptibility of earth formations traversed by the bore hole. In the device disclosed in this patent, a mud sampling channel is provided where the flow of mud is temperature stabilized.